Whenever the fuel cell power plant may be inoperative at below freezing temperatures, whether or not antifreeze coolant is used in a fuel cell power plant, at least product water and membrane humidification must be dealt with in shutting down and restarting the fuel cell power plant. There are various schools of thought concerning steps that may or should be taken in order to avoid damage to the fuel cell power plant due to water expansion when it freezes, and to achieve startup with no water, or with frozen water.
In U.S. Pat. No. 6,596,426 a procedure is set forth to remove all of the water from the water transport plates and from the electrode substrates. This, of course, is deemed favorable to prevent damage from ice. In U.S. patent application Ser. No. 10/390,439 filed Mar. 17, 2003 (now U.S. Pat. No. 6,703,870), that procedure is endorsed as desirable since it leaves the hydrophilic substrates of the electrode support plates empty, thereby being available to store newly-generated product water. In that application, the starting up and running of a fuel cell driving an electrical load is said to be possible for several minutes; however, depending upon all of the attendant conditions, there may be local overheating due to an inadequate cooling capability and membrane dry out.
In U.S. Pat. No. 6,379,827, a procedure is established for filling the electrode substrates with water so as to isolate the membrane from reactant conditions that may occur during startup or shut down. In that patent, as shown in FIG. 1 herein, a fuel cell power plant 10 illustrates one cell, which however is repeated throughout a fuel cell stack. Each fuel cell includes fine pore, flow field plates 12 and 14 used to form the reactant and coolant or water circulating flow fields when a series of cells are pressed together in a stack. These flow field plates are sometimes also referred to as “water transport plates”. The system 10 has a membrane electrode assembly (MEA) 16 which includes a polymer electrolyte, proton exchange membrane (PEM) 18, an anode catalyst layer 20 on the anode side of the membrane 18, and a cathode catalyst layer 24 on the cathode side of the membrane 18. An anode support plate 21 is positioned adjacent the anode catalyst layer 20, and a cathode support plate 25 is positioned adjacent the cathode catalyst layer 24. As depicted in FIG. 1, the anode support plate 21 includes a hydrophilic (wettable) anode substrate 22 in accordance with the invention. The cathode support plate 25 also includes a hydrophilic cathode substrate 26 and further, a hydrophobic (non-wettable) cathode diffusion layer (contact bi-layer) 27.
A set of the flow field plates 12 and 14 of adjacent cells are provided in back-to-back relationship on each side of the MEA 16, outward of the anode and cathode support plates 21, 25. Opposite surfaces of the plate are provided with a pattern of projections 28 and 30 which form a network of grooves 32 and 34 on opposite sides of the plate 12. The grooves 32 form a portion of a coolant water flow field 36 in the stack, and the grooves 34 form a cathode or oxidant reactant flow field 38 for each cell of the stack. The plate 14 is also formed with projections 40 and 42, and a network of grooves 44 and 46 on its opposite surfaces. The grooves 44 form a portion of the water coolant flow field 36, and the grooves 46 from an anode or fuel reactant flow field 48 for each cell in the stack. For simplicity of illustration, the cathode and the anode flow fields 38, 48 are shown in FIG. 1 as extending in the same direction, but preferably extend in directions perpendicular to each other. Moreover, the projections 40 and 42 and the grooves 44 and 46 may be of a variety of shapes and configurations other than as shown.
All of the anode reactant flow fields 48 in the system 10 are supplied with a hydrogen gas reactant from a pressurized fuel source 50, such as a supply tank. The fuel source 50 may be a fuel processing system, which, as is known to those skilled in this art, converts a hydrocarbon fuel such as natural gas or gasoline into a hydrogen rich stream. The hydrogen reactant flows from the supply tank 50 to the anode flow fields 48 through a supply line 52, and the anode exhaust leaving the anode flow fields 48 is directed by an exhaust line 53 to a burner (not shown) or recycle loop (not shown). The pressure of hydrogen flowing through the supply line 52 is controlled by a controller 54 which controllably adjusts a supply valve 56. The pressure of hydrogen flowing through the supply line 52 may additionally be controlled by a pressure regulator 58. The pressures of the reactant in the anode flow field 48 and the reactant in the cathode flow field 38 are preferably about the same level. The present example operates with reactants at near ambient pressure, but higher pressure operation is also possible.
All the cathode flow fields 38 are supplied with ambient air via an air blower or compressor 60 actuated by the controller, and an air line 62. The oxygen used in the electrochemical reaction is thus derived from a source 61 which may be ambient air or a pressurized oxygen.
Coolant water is circulated through the power plant cell units via line 64. The coolant water passes through the coolant flow fields or passages 36 between the plates 12 and 14. Circulation of the coolant water is driven by a fixed or variable speed pump 66 which is actuated by controller 54. The coolant water circulating loop includes a heat exchanger 68 which lowers the temperature of the water exiting from the coolant passages 36, a valve 78, and a water accumulator 70 which includes an overflow drain line 72, a drain valve 74, a vent line 75 and a level sensor 76 for controllably opening and closing the drain valve. The accumulator 70 is positioned below the cell stack assembly, and the water volume held by the accumulator and coolant channels must be sufficient to fill both substrates 22 and 26 in all of the fuel cells. The substrates 22 and 26 fill at shutdown because of the capillary forces drawing water into their pores. As a result, the accumulator 70 can actually be at a position below the cell stack and the substrates 22 and 26 will still fill. For hydrophilic substrates with pore diameters of about 30 microns, water will rise, or wick, upwardly as much as 26 inches.
The procedure in the aforesaid patent was not offered for use in fuel cells which will be shut down at sub-freezing temperatures. If it were, the problem would be that with the substrates totally occluded with ice, the reactant gases could not reach the membrane, and therefore the fuel cell power plant could not be operated until water in the substrates was melted by some means other than operation of the fuel cell power plant to generate electric current.